Gateway state-mediated, long-range tunnelling in molecular wires (original) (raw)
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Length-Dependant Tunneling And Hopping Mechanism In Molecular Wires
International journal of engineering research and technology, 2013
The temperature-dependant electron transport characteristics of three molecular wires of different molecular lengths belonging to the family of conjugated benzene molecules were studied. In this article, the conductance values for three molecular wires consisting of different number of benzene rings and amounting to different lengths at different temperature were calculated. The percentage change in conductance values were plotted with respect to temperature for each molecular wire in this research work. We concluded that the longest molecular wire of benzene having molecular length of 17.055A 0 showed the most pronounced effect of temperature on conductance, even though this value was much smaller than the value exhibited by shortest molecular wire of length 5.003A 0 . The results demonstrated that the shorter wires showed highly length dependence and temperature invariant conductance, whereas the longest wire exhibited weak length dependant and temperature variant behaviour. This ...
IJERT-Length-Dependant Tunneling And Hopping Mechanism In Molecular Wires
International Journal of Engineering Research and Technology (IJERT), 2013
https://www.ijert.org/length-dependant-tunneling-and-hopping-mechanism-in-molecular-wires https://www.ijert.org/research/length-dependant-tunneling-and-hopping-mechanism-in-molecular-wires-IJERTV2IS70185.pdf The temperature-dependant electron transport characteristics of three molecular wires of different molecular lengths belonging to the family of conjugated benzene molecules were studied. In this article, the conductance values for three molecular wires consisting of different number of benzene rings and amounting to different lengths at different temperature were calculated. The percentage change in conductance values were plotted with respect to temperature for each molecular wire in this research work. We concluded that the longest molecular wire of benzene having molecular length of 17.055A 0 showed the most pronounced effect of temperature on conductance, even though this value was much smaller than the value exhibited by shortest molecular wire of length 5.003A 0. The results demonstrated that the shorter wires showed highly length dependence and temperature invariant conductance, whereas the longest wire exhibited weak length dependant and temperature variant behaviour. This electron transport behaviour was observed to be changing from tunnelling in shorter length wires to the hopping in longer length wires.
Length dependence of the electronic transparence (conductance) of a molecular wire
Europhysics Letters (EPL), 1996
The electronic transparence of a single molecular wire connecting the two electrodes of a metal-insulating-metal nanojunction decreases exponentially with its length. The transparence attenuation can be quite small depending of the homo-lumo gap of the molecule and of electronic interaction of the wire ends with the electrodes. For a 10 nm long polyene connecting two nano-electrodes, a 100 mV bias voltage will lead to a tunnelling current intensity in the 10 pA range.
Molecular Wire Interconnects: Chemical Structural Control, Resonant Tunneling and Length Dependence
VLSI Design, 1998
Molecular wires have several promising features, that would appear to make them ideal for advanced interconnects in nanoscale electronic devices. We discuss several aspects of the linear and nonlinear conductance of molecular wire interconnects. Topics include energy dependence of molecular conductance, resonant tunneling behavior, control of conductance by molecular structure and geometry, length dependence including the tunneling regime energetics. Design rules using molecular interconnects will differ substantially from those with more standard, lithographically structured silicon interconnects. In particular, the dissipation mechanisms will differ, both tunneling and ballistic regimes should be available, coulomb blockade and staircase behavior will be observed (but under differing conditions) and fabrication of gate electrodes is a challenge.
Physical Review B, 2010
We report a first-principles study of quantum transport in a prototype two-terminal device consisting of a molecular nanowire acting as an interconnect between two gold electrodes. The wire is composed of a series of bicyclo͓1.1.1͔pentane ͑BCP͒ cage-units. The length of the wire ͑L͒ is increased by sequentially increasing the number of BCP cage units in the wire from 1 to 3. A two terminal model device is made out of each of the three wires. A parameter free, nonequilibrium Green's function approach, in which the bias effect is explicitly included within a many body framework, is used to calculate the current-voltage characteristics of each of the devices. In the low bias regime that is considered in our study, the molecular devices are found to exhibit Ohmic behavior with resistances of 0.12, 1.4, and 6.5 ⍀ for the wires containing one, two, and three cages respectively. Thus the conductance value, G c , which is the reciprocal of resistance, decreases as e −L with a decay constant ͑͒ of 0.59 Å −1. This observed variation of conductance with the length of the wire is in excellent agreement with the earlier reported exponential decay feature of the electron transfer rate predicted from the electron transfer coupling matrix values obtained using the two-state Marcus-Hush model and the Koopman's theorem approximation. The downright suppression of the computed electrical current for a bias up to 0.4 V in the longest wire can be exploited in designing a three terminal molecular transistor; this molecular wire could potentially be used as a throttle to avoid leakage gate current.
Molecular Wires: Charge Transport, Mechanisms, and Control
Annals of the New York Academy of Sciences, 1998
By molecular wires, one generally means molecular structures that transmit a signal between two termini. We discuss some theoretical models and analysis for electronically conductive molecular wires in which a single molecule conducts charge between two electrodes. This situation resembles both intramolecular non-adiabatic electron transfer, in which electronic tunneling between donor and acceptor is seen, and mesoscopic quantum transport.
Superexchange tunneling conductance in molecular wires
2018
The modified superexchange model is used to derive the expression for nonresonant tunneling conductance mediated by localized and delocalized molecular orbitals associated with the terminal and the interior molecular units respectively. The model is shown to work as long as delocalization of electron density in the chain's molecular orbitals is sustained during the tunneling. The criteria for reduction of the superexchange model of charge tunneling to the flat barrier model are formulated and the parameters of the barrier model (energy gap and effective electron mass) are specified in the terms of inter-site coupling and energy distance from the Fermi level to the delocalized wire's HOMO level. Application of the theory tothe experiment shows that the modified superexchange model is quite appropriate to explain the experimental results in case of the nonresonance tunneling conductance in --(CH$_2)$$_N$--NH$_2$ and HOOC--(CH$_2)$$_N$--COOH molecular wires.
Nanotechnology, 2007
This article describes arylene-ethynylene molecular wires with 7 nm long backbones and thiolated termini. Cyclic voltammetric studies in solution reveal that the reduction waves of the fluorene, 9-[(4-pyridyl)methylene]fluorene and 9-[di(4-pyridyl)methylene]fluorene units which are embedded in the conjugated π -systems endow these wires with n-doping characteristics. An x-ray crystal structure investigation of 2,7-diiodo-9-[bis(4-pyridinium)methylene]fluorene bis(tetrafluoroborate) 8 established that protonation occurs on both nitrogens of this unit. Self-assembled monolayers of the 7 nm wire 2 on gold substrates exhibit symmetrical current-voltage (I -V ) characteristics when contacted by a gold scanning transmission microscope (STM) tip. The dipyridyl functionality of 2 served to obtain a rectifying junction in which the diprotonated cationic wire is the electron accepting component in combination with an adjacent anionic phthalocyanine as the electron-donating layer. This ionic Au-2H 2+ 2 [CuPc(SO − 3 ) 4 (Na + ) n ] 2/(4−n) bilayer assembly exhibits rectification with current ratios of 15-50 at ±1 V. This dramatic change in I -V characteristics upon simple chemical manipulation proves that the conductivity is a property of the wire molecules 2 in the junction. Ab initio calculations suggest that the molecular wires possess useful structural features which allow the conductance of the molecule to be altered by changing the properties of the side groups attached to the fluorene units.
Spatially resolved tunneling along a molecular wire
1999
We have spatially resolved the electronic penetration of metallic electronic states through a molecular wire connected to an atomically clean contact. The molecular wire, which is 0.3 nm wide and 1.7 nm long, was electronically connected on one side, and a scanning tunneling microscope tip was used as a second movable electronic counterelectrode. The results reveal a clear exponential decay in the transparency (conductance) of the wire with distance from the contacted end. Analysis of the data shows that electrons are transported along the molecular wire by virtual resonance tunneling with an inverse decay length of 4 nm 21 , in excellent agreement with theoretical calculations.
We report a combined experimental and theoretical investigation of the length dependence and anchor group dependence of the electrical conductance of a series of oligoyne molecular wires in single-molecule junctions with gold contacts. Experimentally, we focus on the synthesis and properties of diaryloligoynes with n = 1, 2, and 4 triple bonds and the anchor dihydrobenzo[b]thiophene (BT). For comparison, we also explored the aurophilic anchor group cyano (CN), amino (NH 2 ), thiol (SH), and 4-pyridyl (PY). Scanning tunneling microscopy break junction (STM-BJ) and mechanically controllable break junction (MCBJ) techniques are employed to investigate single-molecule conductance characteristics. The BT moiety is superior as compared to traditional anchoring groups investigated so far. BT-terminated oligoynes display a 100% probability of junction formation and possess conductance values which are the highest of the oligoynes studied and, moreover, are higher than other conjugated molecular wires of similar length. Density functional theory (DFT)-based calculations are reported for oligoynes with n = 1−4 triple bonds. Complete conductance traces and conductance distributions are computed for each family of molecules. The sliding of the anchor groups leads to oscillations in both the electrical conductance and the binding energies of the studied molecular wires. In agreement with experimental results, BT-terminated oligoynes are predicted to have a high electrical conductance. The experimental attenuation constants β H range between 1.7 nm −1 (CN) and 3.2 nm −1 (SH) and show the following trend: β H (CN) < β H (NH 2 ) < β H (BT) < β H (PY) ≈ β H (SH). DFT-based calculations yield lower values, which range between 0.4 nm −1 (CN) and 2.2 nm −1 (PY).